Technical Field
The present disclosure relates to fluidizable vanadium based VOx/Ce-γ-Al2O3 catalysts and processes using the catalysts for the cracking of alkanes to olefins, such as hexane to propylene, in the absence of gas phase oxygen.
Description of the Related Art
The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present invention.
Olefins such as ethylene and propylene are important feedstock in the chemical industry. They are used for the production of polyethylene, polypropylene and other industrial intermediates. Conventionally, olefins are obtained by steam cracking and catalytic conversion of ethanol. Steam cracking processes suffer from high production costs due to the use of petroleum feedstock and the energy consumption in the furnace [T. Ren, M. Patel, and K. Blok, “Olefins from conventional and heavy feedstocks: Energy use in steam cracking and alternative processes,” Energy, vol. 31, pp. 425-451, 2006.]. Additionally, ethane crackers as well as the retrofitting of naphtha crackers were developed after the rise of shale oil technology over the past decades [N. Rahimi and R. Karimzadeh, “Catalytic cracking of hydrocarbons over modified ZSM-5 zeolites to produce light olefins: A review,” Appl. Catal. A Gen., vol. 398, no. 1-2, pp. 1-17, 2011; and B. Yilmaz and U. Müller, “Catalytic applications of zeolites in chemical industry,” Top. Catal., vol. 52, pp. 888-895, 2009.—each incorporated herein by reference in its entirety]. Furthermore, olefins are produced from methanol using zeolite processes such as the methanol to olefins (MTO) process. The two commonly used catalysts in this process are ZSM-5 (MFI-type) and SAPO-34 (CHA-type). MFI is a medium pore sized alumina-silicate with ten membered ring pores. MFI is highly selective toward propylene and butylene; however, the yield of short olefins is less than in SAPO-34. SAPO-34 is a silico-alumino-phosphate with small eight-membered ring pores, which has high selectivity towards ethylene but suffers from fast deactivation due to coke formation [D. Chen, K. Moljord, and a. Holmen, “A methanol to olefins review: Diffusion, coke formation and deactivation on SAPO type catalysts,” Microporous Mesoporous Mater., vol. 164, pp. 239-250, 2012; and S. Askari, R. Halladj, and M. Sohrabi, “Methanol conversion to light olefins over sonochemically prepared SAPO-34 nanocatalyst,” Microporous Mesoporous Mater., vol. 163, pp. 334-342, 2012; and G. Liu, P. Tian, Q. Xia, and Z. Liu, “An effective route to improve the catalytic performance of SAPO-34 in the methanol-to-olefin reaction,” J. Nat. Gas Chem., vol. 21, no. 4, pp. 431-434, 2012.—each incorporated herein by reference in its entirety]. Therefore, in general zeolites have deactivation problems and generally low selectivity to olefins (i.e. less than 50%) [Mamedov, E. A.Corberfin, V. Cortds “Oxidative dehydrogenation of lower alkanes on vanadium oxide-based catalysts. The present state of the art and outlooks” Appl. Catal. A Gen., vol. 127, pp. 1-40, 1995; and J. Li, Y. Wei, G. Liu, Y. Qi, P. Tian, B. Li, Y. He, and Z. Liu, “Comparative study of MTO conversion over SAPO-34, H-ZSM-5 and H-ZSM-22: Correlating catalytic performance and reaction mechanism to zeolite topology,” Catal. Today, vol. 171, no. 1, pp. 221-228, 2011; and J. Lefevere, S. Mullens, V. Meynen, and J. Van Noyen, “Structured catalysts for methanol-to-olefins conversion: a review,” Chem. Pap., vol. 68, no. 9, pp. 1143-1153, 2014.]. Catalytic cracking of hexane is another promising process to produce short olefins, in these processes many types of catalyst have been used including metal oxides as well as zeolites. Zeolites have been tested for n-hexane cracking to propylene, such as the zeolite MCM-22 with various Si/Al ratios [Y. Wang, T. Yokoi, S. Namba, J. N. Kondo, and T. Tatsumi, “Catalytic cracking of n-hexane for producing propylene on MCM-22 zeolites,” Appl. Catal. A Gen., 2014.—incorporated herein by reference in its entirety]. Propylene selectivity was found to be 40% at a Si/Al ratio of 62. ZSM-5 zeolites were also investigated using methanol coupling [F. Chang, Y. Wei, X. Liu, Y. Qi, D. Zhang, Y. He, and Z. Liu, “An improved catalytic cracking of n-hexane via methanol coupling reaction over HZSM-5 zeolite catalysts,” Catal. Letters, vol. 106, no. February, pp. 171-176, 2006.—incorporated herein by reference in its entirety]. Methanol contributed positively to the reaction by decreasing the activation energy which ultimately led to an increased olefins yield. Another study investigated the cracking of hydrocarbons over a ZSM-5 catalyst with different Si/Al ratios [B. Lücke, A. Martin, H. Gunschel, and S. Nowak, “CMHC: coupled methanol hydrocarbon cracking,” Microporous Mesoporous Mater., vol. 29, pp. 145-157, 1999.—incorporated herein by reference in its entirety]. The catalyst was also modified using iron (Fe) to decrease catalyst deactivation; this increased the yield of olefins to as high as 305. In addition, metal oxides, such as MoO2 were considered as potential catalysts for n-hexane cracking obtaining a total olefins yield of 85% [J. H. Song, P. Chen, S. H. Kim, G. a. Somorjai, R. J. Gartside, and F. M. Dautzenberg, “Catalytic cracking of n-hexane over MoO2,” J. Moir. Catal. A Chem., vol. 184, pp. 197-202, 2002.—incorporated herein by reference in its entirety]. Catalytic oxidative cracking is a potential alternative to steam cracking due to factors including: (i) an exothermic oxidation reaction, (ii) an adiabatic process, and (iii) a reduced coke formation. Although, for the non-catalytic pyrolysis of hexane at 750° C., the oxygen feed was found to increase reaction rate and give a conversion of 85% with an olefin selectivity of 59% and ethylene as the major product [Xiaoyin Chena, Yong Liva, Guoxing Niub, Zhuxian Tanga, Maiying Biana, Adi Hea, “High temperature thermal stabilization of alumina modified by lanthanum species” React. Kinet. Catal. Lett, vol. 81, no. 2, pp. 203-209, 2001.—incorporated herein by reference in its entirety]. The presence of oxygen allows the cracking to proceed in an autothermal way, where the exothermic reaction provides the heat for the cracking reaction. Liu, et al. conducted a comprehensive study of homogeneous gas phase versus heterogeneous catalytic oxidative cracking of hexane at a temperature of 700° C. [H. X. X. Liu, W. Li, H. Zhu, Q. Ge, Y. Chen, “Light Alkenes Preparation by the Gas Phase Oxidative Cracking or Catalytic Oxidative Cracking of High Hydrocarbons”, Catal. Lett., vol. 94, no. 1-2, pp. 31-36, 2004.—incorporated herein by reference in its entirety]. Amongst the catalysts tested, 0.25 wt % Li/MgO showed the best performance (64 mol % conversion of hexane and 67 mol % selectivity to olefins). However, due to the high reaction temperature, the gas phase reaction had the major role and the presence of catalyst had no major influence on conversions of hexane and yields of olefins. Li/Mg0 catalysts with different promoters (i.e. MoO3, Bi2O3, V2O5) were tested for oxidative cracking of n-hexane and the olefin selectivity obtained was up to 50% [C. Boyadjian, B. Van Der Veer, I. V. Babich, L. Lefferts, and K. Seshan, “Catalytic oxidative cracking as a route to olefins: Oxidative conversion of hexane over MoO3-Li/MgO,” Catal. Today, vol. 157, pp. 345-350, 2010.—incorporated herein by reference in its entirety].
Oxidative cracking of n-hexane over supported metal oxides is a potential synthetic route for the production of short olefins. Studies have shown that the presence of metal oxide catalysts during cracking enhances yield of olefins. This observation can be explained qualitatively with a mechanism that includes activation of the alkane on the catalyst generating alkyl radicals that undergo a radical-chain mechanism in the gas phase. In this mechanism oxygen has two functions. First, the presence of small amounts of oxygen influences the radical gas phase chemistry significantly because the type and concentration of the chain propagator radicals are greatly increased. At higher oxygen partial pressures the radical chemistry is only slightly influenced by the increasing oxygen concentration. The second function of oxygen is to facilitate the removal of hydrogen from the surface OH− species that are formed during the activation of alkane on the catalyst. Therefore, it has been shown that the oxidative conversion of propane over Li/MgO catalysts follows a mixed heterogeneous-homogeneous radical chemistry where the catalyst acts as an initiator [L. Leveles, K. Seshan, J. A. Lercher, and L. Lefferts, “Oxidative conversion of propane over lithium-promoted magnesia catalyst—I. Kinetics and mechanism,” J. Catal., vol. 218, pp. 296-306, 2003.—incorporated herein by reference in its entirety]. Hence, the mentioned mechanism was used to study n-hexane cracking over Li/MgO modified catalysts, and it was proposed that hexane is activated before the cracking reaction takes place. The study also showed that the catalyst increases the production of olefins via dehydrogenation. Oxidative cracking of n-hexane to olefins is a promising process; however, all previous work was done using fixed-bed reactors and gas phase oxygen as a co-reactant for the purpose of dehydrogenation. This setup induces COx formation by combustion due to the presence of oxygen and therefore a lower yield of olefins is obtained.
In view of the forgoing, one aspect of the present invention is to provide fluidizable oxidative cracking catalysts comprising vanadium oxide catalytic species using a mixed Ce-γ-Al2O3 as support material. The physiochemical characterization of the catalysts offers an examination of the VOx monovanadate and polyvanadate surface species on the support, the catalyst's stability, level of acidity and metal-support interactions. A further aim of the present disclosure is to provide methods for producing these VOx/Ce—Al2O3 catalysts. An additional aim of the present disclosure is to provide methods for the oxidative cracking of alkanes, such as hexane, to produce one or more olefins, such as ethylene, propylene, and/or butylene employing the lattice oxygen of these VOx/Ce—Al2O3 catalysts. These catalysts present relatively high acidity enhancing the cracking reaction, and simultaneously the catalyst lattice oxygen allows dehydrogenation of alkanes to olefins to take place (FIG. 1). These methods may be performed in a gas phase oxygen free environment under fluidized bed reaction conditions that enhance catalyst-feed contact at different temperatures and reaction times accomplishing high alkane conversion and high olefins product selectivity over COx combustion products.